Are there any pokemon or features you don't get in Diamond or Pearl?
The main differences are you get Giratina first and can get both Dialga and Palkia later. Another difference is the appearence of the Distortion World where you meet Giratina. Also the order you face the gym leaders is slightly altered. One of the best benefits of Platinum is the Battle Frontier.
Comparing these 2 descriptions will give you all the differences.
Platinum is a lot faster than diamond/pearl, and the gyms have different features.